This invention relates generally to pollution control systems for automotive vehicles, and more particularly to devices especially adapted to minimize the inadvertent release of fuel vapors from the vehicle engine and fuel system directly into the atmosphere.
Within the past several years a number of attempts have been made to limit the evaporation of gasoline from automotive fuel systems, especially the evaporation which occurs while the vehicles are idle. Special fuel tank caps have been employed in order to reduce or eliminate fumes that would otherwise escape. In addition, recently there has been developed what is generally known as an evaporative emission control system that involves the use of a charcoal-filled canister which is connected, through a vapor line, to the fuel tank such that gasoline vapors from the tank are channelled into the canister and absorbed and partially condensed in the charcoal. The vapor line in this system is connected to an inlet port located at the top of the canister. A second port on the canister is also provided, known as a "purge" port, from which a line extends to the intake manifold or carburetor of the vehicle's engine. When the vehicle is started, condensed fuel and vapor that is stored in the charcoal is released and sucked into the engine to be burned.
The vapor storage and recovery canisters that have previously been employed generally were either of the closed or else the open bottom types. In the open bottom canister, there is a series of relatively large air-intake passages that can admit fresh air from the engine compartment during operation of the engine. This fresh air flow picks up most of the stored fuel from the charcoal and carries it to either the carburetor or else to the intake manifold. Additional fumes which enter the canister during engine operation are purged continuously, leaving the charcoal in a relatively "dry" condition, ready to adsorb fuel after the engine is shut down. Usually there is some type of filter material, such as fiberglass wadding, at the fresh air intake area so as to eliminate any tendency for dirt or dust to be drawn into the canister.
Some canisters have been designed to draw vapors from both the fuel tank and the carburetor float bowl during engine shutdown, and to purge such fuel vapors or their condensate during vehicle operation. Other arrangements, such as that described above, involve recovery of fumes from only the vehicle's fuel tank.
Where it is desired to draw vapors from both the fuel tank and the carburetor float bowl, there are usually required control valves in order to enable the float bowl to vent into the canister when the engine is not running, while still not interfering with the normal operation thereof. In addition, fuel tank pressure valves have been incorporated in certain vehicles, these functioning to restrict venting of the tank during idle periods, and to increase the capability of venting into the canister during engine operation. Various other arrangements have also been proposed and produced.
The major problem with evaporative emission control systems of the type employing activated carbon or activated charcoal as adsorptive materials housed in a vapor canister, is that under conditions of low ambient temperature the fuel that has been adsorbed exhibits a reluctance to be evaporated and purged from the canister. It has been determined that with adsorptive substances currently being employed, satisfactory release of the stored fuel will occur at room temperatures and above; however, when the temperatures fall much below these values, the efficiency of the system suffers significantly. The ability of the carbon to release the fuel is poor until the canister heats up.
Such units are therefore generally located in the engine compartment of the vehicle, and with the exception of extremely cold weather, eventually the desired canister temperatures are reached. However, until this occurs the fuel occupying the canister tends to remain there, as opposed to being promptly drawn into the intake manifold so as to be burned off.